1. Technical Field of the Invention
The present invention relates to driver circuits for semiconductor power switching devices, and particularly to power switch controllers or drivers having improved current limiting protection for the switches.
2. Relevant Art
High power semiconductor switches, typically MOSFETS, are employed in high current power supplies, motor drives, and other applications. In such equipment, the switches are gated on and off by driver circuits. The driver circuits also include sensing and protective circuits for the power switches, and are often referred to as “intelligent”.
The driver circuits often require voltages higher than the supply voltage, and a conventional way of providing these voltages is by use of a so-called charge pump. Broadly stated, as known by those skilled in the art, a charge pump operates by effectively transferring charge built-up on a first capacitor to a second capacitor. In addition, current limiting circuits are utilized to limit the current through the power switch.
FIG. 15 shows an example of a driver-current limiter circuit according to conventional practice, generally designated at 18. Here, op-amps 20 and 22 respectively provide gate control signals for a power MOSFET switch 24 and a sensing MOSFET 26. These may be combined as a MOSFET device including switch 24 as a current carrying cell and switch 26 as a sensing cell. A comparator op-amp 28 receives a reference voltage VREF at a direct input, and a current sense signal across a resistor 30 connected between the source terminal of MOSFET 26 and the source voltage VSS at its inverting input. An operating voltage VCP for op-amps 20 and 22 is provided by a single charge pump (not shown), while op-amp 28 is connected in common with MOSFETS 24 and 26 to a supply voltage VDD.
In the illustrated circuit, as long as the source of switch 26 is at least about 1.5 volts below the drain voltage VDD, current limiting is provided by comparator op-amp 28. However, when the drain-source voltage is less than approximately 1.5 volts, current limiting is not operative, and significant current overshoots beyond the desired current limit are possible. The reason for this is that op-amp 28 cannot operate as the output voltage nears the supply voltage VDD.
FIG. 16 shows the relationship between VDS and the MOSFET output current, and the current limit ILIM determined by VDS/RDS-ON. As may be seen, when VDS is below 1.5 volts, an overshoot (shown at 32) occurs and the current exceeds the current limit ILIM. When VDS equals or exceeds about 1.5 volts, the current limiting operates to clamp the current level to ILIM.
In the past, the RDS-ON of the typical MOSFET was in 150 milliohm range, and the current limits were adequate to make the overshoots tolerable. However, the newer generation of power FETS exhibit RDS-ON approximately one-tenth the former level, e.g., approximately 13 milliohms, and the peak currents can be significantly higher.
This may also be seen in FIG. 16. Since the resistance RDS-ON is inversely related to the slope, if the slope increases, corresponding to lower RDS-ON, the amount of overshoot below a VDS of about 1.5 volts can increase significantly, as shown by the dashed lines. Thus, a need clearly exists to provide a circuit architecture which ensures that current limiting occurs at VDS voltages below about 1.5 volts. The present invention is intended to meet this need.